There are known many types of thermal detectors, such as thermoelectric detectors, bolometers, pyroelectric detectors and the like, which are based on the measurement of a change of some temperature-dependent property of the detector material which is induced by the radiation to be measured, and which is transformed into an output signal which is measured.
Thermocouples and thermopiles are used for the detection and measurement of electromagnetic radiation, especially in the I.R. range. These are based on the thermoelectric e.m.f. induced by the rise of temperature caused by said radiation. These devices are used for low chopping frequencies in the range of subaudio and low audio frequencies. As for high frequencies, the sensitivity of such devices drops rapidly and approaches zero. Fast thermocouples and bolometers, for the 10.sup.7 to 10.sup.9 Hz range have been proposed by Block et al., IEEE Q.E. (1973) 1044 and Contreras et al., Appl. Phys. Lett. 17, (1970) 450. Such devices make use of thin films deposited on a heat-conducting substrate. The known devices suffer from the drawback of low responsivity (about 10.sup.-.sup.5 to 10.sup.-.sup.4 V/W). With pyroelectric detectors, the response time can be varied by the provision of a resistor connected in series with the detector, see Laser Focus, July 1973. An increase of frequency with such devices brings about a moderate decrease in sensitivity. The sensitivity of pyroelectric detectors decreases as the square root of the response time.